Excited‐State Evolution of Keto‐Carotenoids after Excess Energy Excitation in the UV Region

Abstract

Carotenoids are molecules with rich photophysics that are in many biological systems involved in photoprotection. Yet, their response to excess energy excitation is only scarcely studied. Here we have explored excited state properties of three keto‐carotenoids, echinenone, canthaxanthin and rhodoxanthin after excess energy excitation to a singlet state absorbing in UV. Though the basic spectral features and kinetics of S2, hot S1, relaxed S1 states remain unchanged upon UV excitation, the clear increase of the S* signal is observed after excess energy excitation, associated with increased S* lifetime. A multiple origin of the S* signal, originating either from specific conformations in the S1 state or from a non‐equilibrated ground state, is confirmed in this work. We propose that the increased amount of energy stored in molecular vibrations, induced by the UV excitation, is the reason for the enhanced S* signal observed after UV excitation. Our data also suggest that a fraction of the UV excited state population may proceed through a non‐sequential pathway, bypassing the S2 state.

UV Excitation of Carotenoid Binding Proteins OCP and HCP: Excited‐State Dynamics and Product Formation

“…This is caused by increased S* signal in the 600-630 nm region, which is induced by UV excitation, an effect reported earlier for carotenoids in solution. [41][42] Thus, the 510 nm peak of the ground state bleaching that is off the absorption maximum of both HCPs (Figure 1) is rather caused by increased excited state absorption that significantly overlaps with ground state bleaching in the UV-excited spectra.…”

“…We have applied global fitting analysis to the buffer-subtracted data obtained after UV excitation, the results are shown in Figure 5. Due to limited time resolution of the UV-excited experiment (~250 fs) [42] the initial EADS (evolution-associated difference spectra) is a mixture containing contributions from the initially excited state and states populated within the first 300 fs. The positive signal above 1000 nm in the first EADS indicates involvement of the S 2 state during relaxation of the UV-excited state; the distinct band due to S 2 À S m transition is readily identified especially for OCP (Figure 5C).…”

“…[38][39] Recent studies suggest its involvement in excess energy dissipation, [40] which is also supported by the increase of S* signal upon UV excitation. [41][42] Since the S* has been proposed to be the key state initializing the OCP photocycle [15] and since the S* signal can be enhanced by UV excitation, it is natural to ask whether UV excitation of OCP may increase the efficiency of formation of the initial photoproduct. Studies exploring the carotenoid excited state dynamics after UV excitation are scarce and so far limited only to carotenoids in solution.…”

“…Recently, however, UV excitation study of keto-carotenoids including canthaxanthin that is often found in OCP/HCP showed that the S* amplitude is enhanced by UV excitation also in these carotenoids. [42] Further, because it is generally accepted that the first step of the OCP photocycle is breaking the hydrogen bond between carotenoid and Trp/Tyr in CTD, [13,15,[43][44][45] excitation of Trp may also cause disruption of hydrogen bond, potentially leading to the same effect as carotenoid excitation. Direct excitation of Trp and Tyr may also shed light on their yet-unknown role in HCPs.…”

Structure elucidation of the novel carotenoid gemmatoxanthin from the photosynthetic complex of Gemmatimonas phototrophica AP64

“…Similar characteristics may have also predicted in the carotenoid bacteriorubixanthinal present in the photosynthetic complexes of Erythrobacter species 35 . Numerous studies have been published on the energy transfer e ciency of carbonyl carotenoids to the BChl a in photosynthetic complexes 36,37 . The energy transfer e ciency of rhodopinal to BChl a found to be ~100% 38 and similarly, for okenone the energy transfer e ciency is ~ 95 % 39 .…”